Gas discharge lamp with down conversion luminophore

ABSTRACT

A gas discharge lamp fitted with a gas discharge vessel filled with a gas filling is suitable for a gas discharge which emits VUV radiation, with a luminophore coating containing a down conversion luminophore and with means for igniting and maintaining a gas discharge in which the down conversion luminophore has in a host lattice a pair of activators of the a first lanthanoid ion and a second lanthanoid ion and a sensitizer selected from the group of the cerium (III) ion, praseodymium (III) ion, neodymium (III) ion, samarium (III) ion, europium (III) ion, gadolinium (III) ion, terbium (III) ion, dysprosium (III) ion, holmium (III) ion, erbium (III) ion, thulium (III) ion, ytterbium (III) ion and lutetium (III) ion, is environmentally friendly and has a high lamp efficiency η lamp .  
     The invention also concerns a down conversion luminophore which in a host lattice has a pair of activators of a first lanthanoid ion and a second lanthanoid ion and a sensitizer selected from the group of the cerium (III) ion, praseodymium (III) ion, neodymium (III) ion, samarium (III) ion, europium (III) ion, gadolinium (III) ion, terbium (III) ion, dysprosium (III) ion, holmium (III) ion, erbium (III) ion, thulium (III) ion, ytterbium (III) ion and lutetium (III) ion.

[0001] The invention relates to a gas discharge lamp fitted with a gasdischarge vessel filled with a gas suitable for supporting a gasdischarge emitting VUV radiation, with a luminophore coating containinga down conversion luminophore and with means for igniting andmaintaining a gas discharge.

[0002] Conventional fluorescent lamps are mercury gas discharge lamps,the light emission of which is based on mercury low pressure gasdischarge. A mercury low pressure gas discharge emits radiation mainlyin the near UV with a maximum at 254 nm which is converted into visiblelight by UV luminophores.

[0003] The mercury gas discharge lamp has a refined technology and withregard to the lamp efficiency η_(lamp) can only be matched or exceededwith difficulty by other lamp technologies.

[0004] The mercury in the gas filling is however increasingly regardedas an environmentally harmful and toxic substance which should beavoided as far as possible in modern mass production because ofenvironmental risks in use, production and disposal. Therefore for sometime efforts have been concentrated on the development of alternativelamp technologies.

[0005] One of the mercury-free or low-mercury alternatives to theconventional mercury gas discharge lamp is the xenon low pressure gasdischarge lamp which has a gas filling containing mostly xenon. A gasdischarge in a xenon low pressure gas discharge lamp emits vacuumultraviolet radiation (VUV radiation) in contrast to the UV radiation ofthe mercury discharge. The VUV radiation is generated by excimers e.g.Xe₂* , and is a molecular band radiation with a broad spectrum in therange about 172 nm. Using this lamp technology discharge efficienciesη_(dis) of 65% are achieved.

[0006] Another advantage of the xenon low pressure gas discharge lamp isthe short response time of the gas discharge which makes it useful as asignal lamp for automobiles, as a lamp for copier or fax devices and asa water disinfection lamp.

[0007] However although the xenon low pressure gas discharge lamp hasachieved a discharge efficiency η_(dis) which is comparable to that ofthe mercury gas discharge lamp, the lamp efficiency η_(lamp) of thexenon low pressure gas discharge lamp is still clearly lower than thatof the mercury gas discharge lamp.

[0008] In principle the lamp efficiency η_(lamp) consists of thecomponents discharge efficiency η_(dis), luminophore efficiencyη_(phos), the proportion of the generated visible light which leaves thelamp η_(esc) and the proportion η_(vuv) of UV radiation generated by theluminophore:

η_(lamp)=η_(dis)·η_(phos)·η_(esc)·η_(vuv).

[0009] One handicap of the conventional xenon low pressure gas dischargelamp lies in the conversion, ineffective in principle, of an energy-richVUV photon with wavelength of around 172 nm into a comparatively lowenergy photon from the visible spectrum of 400 nm to 700 nm through theluminophore coating of the lamp. Even if the quantum efficiency of theluminophore is close to 100%, by conversion of a VUV photon into avisible photon, on average 65% of the energy is lost due toradiationless transition.

[0010] Surprisingly however it has already been possible to develop VUVluminophores which achieve a quantum efficiency of more than 100% forconversion of VUV photons into visible photons. This quantum efficiencyis achieved in that a VUV quantum with an electron energy of 7.3 eV isconverted into two visible quantums with an electron energy of 2.5 eV.Such luminophores for xenon low pressure gas discharge lamps are knownfor example from René T. Wegh, Harry Donker, Koentraad D. Oskam, AndriesMeijerink “Visible Quantum Cutting in LiGdF₄:Eu³⁺ throughDownconversion” Science 283, 663.

[0011] In analogy to the multiphoton luminophores known for some time,which through “up conversion” generate from two visible long-wavephotons one short-wave photon, these new luminophores, which generatefrom one short-wave photon two long-wave photons, are known as downconversion luminophores.

[0012] But although the quantum efficiency of the known down conversionluminophores is high, this does not mean that consequently theluminophore efficiency η_(phos) is high. The luminophore efficiencyη_(phos) is influenced not only by the quantum efficiency but also bythe capacity of the luminophore to absorb the VUV radiation to beconverted. The absorption capacity of the known down conversionluminophores is however quite low. Too much energy is lost throughundesirable absorption in the lattice and hence the occupation of theexcited states reduced.

[0013] It is an object of the present invention to develop a gasdischarge lamp fitted with a gas discharge vessel filled with a gassuitable for gas discharge which emits VUV radiation, with a luminophorecoating which contains a down conversion luminophore and with means forigniting and maintaining a gas discharge, and with improved efficiency.

[0014] According to the invention this object is achieved by a gasdischarge lamp fitted with a gas discharge vessel filled with a gasfilling suitable for supporting a gas discharge emitting VUV radiation,with a luminophore coating containing a down conversion luminophore andwith means for igniting and maintaining a gas discharge, in which thedown conversion luminophore contains a pair of activators of a firstlanthanoid ion and a second lanthanoid ion and a sensitizer selectedfrom the group of the cerium (III) ion, praseodymium (III) ion,neodymium (III) ion, samarium (III) ion, europium (III) ion, gadolinium(III) ion, terbium (III) ion, dysprosium (III) ion, holmium (III) ion,erbium (III) ion, thulium (III) ion, ytterbium (III) ion and lutetium(III) ion.

[0015] Particularly advantageous effects in relation to the state of theart are obtained by the invention if the first lanthanoid ion is thegadolinium (III) ion and the second lanthanoid ion is selected from theholmium (III) ion and the europium (III) ion.

[0016] As part of the present invention it is preferred for the downconversion luminophore to contain as the first lanthanoid ion thegadolinium (III) ion and as the second lanthanoid ion the holmium (III)ion and a co-activator selected from the group of the terbium (III) ion,ytterbium (III) ion, dysprosium (III) ion, europium (III) ion, samarium(III) ion and manganese (II) ion.

[0017] It can also be preferred that the host lattice of the downconversion luminophore is a fluoride.

[0018] It is particularly preferred that the down conversion luminophorecontains the first lanthanoid ion in a concentration of 10.0 to 99.98mol %, the second lanthanoid ion in a concentration of 0.01 to 30.0 mol% and the sensitizer in a concentration of 0.01 to 30 mol %.

[0019] According to one embodiment of the gas discharge lamp accordingto the invention the down conversion luminophore contains the sensitizerin a concentration of 0.5 mol %.

[0020] In a different embodiment of the invention the down conversionluminophore contains the co-activator in a concentration of 0.01 to 30mol %.

[0021] According to a further embodiment of the invention the downconversion luminophore contains a co-activator in a concentration of 0.5mol %.

[0022] The invention also relates to a down conversion luminophore whichcontains in a host lattice a pair of activators of a first lanthanoidion and a second lanthanoid ion and a sensitizer selected from the groupof the cerium (III) ion, praseodymium (III) ion, neodymium (III) ion,samarium (III) ion, europium (III) ion, gadolinium (III) ion, terbium(III) ion, dysprosium (III) ion, holmium (III) ion, erbium (III) ion,thulium (III) ion, ytterbium (III) ion and lutetium (III) ion.

[0023] The luminophore is characterized by a high quantum efficiency, ahigh absorption of VUV photons and also a high chemical resistance andis therefore particularly suited for commercial applications includingin plasma screens. Such a luminophore can advantageously also be usedfor signaling lamps in motor vehicles.

[0024] The invention is now described in more detail.

[0025] A gas discharge lamp according to the invention comprises a gasdischarge vessel with a gas filling and with at least one wall which hasa surface that is at least partially transparent to visible radiationwith a luminophore layer. The luminophore coating contains a luminophorepreparation with an inorganic crystalline host lattice which hasobtained its luminosity from activation through an activator pair of afirst and a second lanthanoid ion. The down conversion luminophore issensitized by a sensitizer from the group of the cerium (III) ion,praseodymium (III) ion, neodymium (III) ion, samarium (III) ion,europium (III) ion, gadolinium (III) ion, terbium (III) ion, dysprosium(III) ion, holmium (III) ion, erbium (III) ion, thulium (III) ion,ytterbium (III) ion and lutetium (III) ion. Also the gas discharge lampis fitted with an electrode structure to ignite the gas discharge andwith further means to ignite and maintain the gas discharge.

[0026] Preferably the gas discharge lamp is a xenon low pressure gasdischarge lamp. Various types of xenon low pressure gas discharge lampsare known which differ in the ignition of the gas discharge. Thespectrum of gas discharge first contains a high proportion of VUVradiation invisible to the human eye which is converted into visiblelight in the coating of VUV luminophores on the inside of the gasdischarge vessel and then radiated.

[0027] The term “vacuum ultraviolet radiation” below also refers toelectromagnetic radiation with a maximum emission in a wavelength rangebetween 145 and 185 nm.

[0028] In a typical construction for the gas discharge lamp thisconsists of a cylindrical glass lamp bulb filled with xenon, on the wallof which on the outside is arranged a pair of strip-like electrodeselectrically insulated from each other. The strip-like electrodes extendover the entire length of the lamp bulb, where their long sides lieopposite each other leaving two gaps. The electrodes are connected tothe poles of a high voltage source operated with an alternating voltageof the order of 20 kHz to 500 kHz such that an electric discharge formsonly in the area of the inner surface of the lamp bulb.

[0029] When an alternating voltage is applied to the electrodes, in thexenon-containing filler gas a silent electrical discharge can beignited. As a result in the xenon are formed excimers i.e. moleculeswhich consist of an excited xenon atom and a xenon atom in the basicstate.

Xe+X*=Xe ₂*

[0030] The excitation energy is emitted again as VUV radiation with awavelength of λ=170 to 190 nm. This conversion from electron energy intoUV radiation is highly efficient. The generated VUV photons are absorbedby the luminophores of the luminophore layer and the excitation energypartly emitted again in the longer wavelength range of the spectrum.

[0031] In principle for the discharge vessel a multiplicity of forms arepossible such as plates, single tubes, coaxial tubes, straight,U-shaped, circular curved or coiled, cylindrical or other shapedischarge tubes.

[0032] As a material for the discharge vessel quartz or glass types areused.

[0033] The electrodes consist of a metal e.g. aluminum or silver, ametal alloy or a transparent conductive inorganic compound e.g. ITO.They can be formed as a coating, as an adhesive foil, a wire or a wiremesh.

[0034] The discharge vessel is filled with a gas mixture containing anoble gas such as xenon, krypton, neon or helium. Gas fillings whichmainly consist of oxygen-free xenon at low gas pressure, e.g. 2 Torr,are preferred. The gas filling can also contain a small quantity ofmercury in order to maintain a low gas pressure during discharge.

[0035] The inner wall of the gas discharge vessel is coated partly orfully with a luminophore coating which contains one or more luminophoresor luminophore preparations. The luminophore layer can also containorganic or inorganic binding agents or a binding agent combination.

[0036] The luminophore coating is preferably applied to the inner wallof the gas discharge vessel as a substrate and can comprise a singleluminophore layer or several luminophore layers, in particular doublelayers of a base and a cover layer. A luminophore coating with base andcover layer allows the quantity of down conversion luminophore in thecover layer to be reduced and in the base layer a less costlyluminophore to be used. The base layer preferably contains as aluminophore a calcium-halophosphate luminophore selected to achieve thedesired shade of the lamp.

[0037] The cover layer contains the down conversion luminophore whichthus forms an essential part of the VUV radiation generated by gasdischarge to be converted directly into the required radiation in thevisible range.

[0038] An essential feature of the down conversion luminophore accordingto the invention is that it contains a pair of activators of a first anda second lanthanoid ion and a sensitizer in a host lattice.

[0039] The first lanthanoid ion of the activator pair is preferably thegadolinium (III) ion, the second lanthanoid ion of the activator paircan be selected from the holmium (III) ion and the europium (III) ion.

[0040] The sensitizer is selected from the group of the cerium (III)ion, praseodymium (III) ion, neodymium (III) ion, samarium (III) ion,europium (III) ion, gadolinium (III) ion, terbium (III) ion, dysprosium(III) ion, holmium (III) ion, erbium (III) ion, thulium (III) ion,ytterbium (III) ion and lutetium (III) ion. Generally these ions arealso known according to their electron configuration as 4f^(n) ions.

[0041] The sensitizer reinforces the sensitivity of the down conversionluminophore for VUV radiation and makes it less wavelength-dependent. Ithas a high self-absorption in the required VUV range of 100 to 200 nmwhich essentially lies above the self-absorption of the non-sensitizeddown conversion luminophores at 183, 195 and 202 nm. The transmission ofexcitation energy to the activator pair carries losses as latticeinterference causes the excitation states passing through the matrix toemit energy to the matrix in the form of heat oscillations. The reducedabsorbed excitation energy is then passed to the activator and triggersthe down conversion mechanism. The down conversion luminophoreluminesces more intensely as it has been “sensitized” by the sensitizerto luminescence capacity under VUV radiation.

[0042] The down conversion luminophore can also contain a co-activator.The co-activator is selected from the group of the trivalent ions ofterbium, ytterbium, dysprosium, europium and samarium and the bivalentions of manganese. The pair of activators of a first lanthanoid ion anda second lanthanoid ion and the co-activator ion co-operate in thesequential emission of the photons with which the luminophore generatesmore than one visible photon from a VUV photon.

[0043] The excitation mechanism can take place by the ⁸S-⁶G excitationof the gadolinium (III) ion which follows a cross relaxation transitionbetween the Gd (III) ion and the holmium (III) ion or europium (III)ion. Due to the cross relaxation transition the gadolinium (III) ionchanges from the ⁶G-state to the ⁶P-state, due to the released energythe holmium (III) ion changes from the ⁵I₈-state to the ⁵F₅-state or theeuropium (III) ion from the ⁷F₀-state to the ⁵D₀-state. The holmium(III) ion or the europium (III) ion then emits a visible photon, theenergy of which corresponds to the transition from ⁵F₅ to ⁵I₈ or ⁵D₀ to⁷F₁.

[0044] After an energy transfer from the ⁶P state of the gadolinium(III) ion to the co-activator, this also emits a visible photon.

[0045] The host lattice of the down conversion luminophore can consistof inorganic material such as fluoride, oxide, halogenide, aluminate,gallate, phosphate, borate or silicate which is doped with a few percentof both activators. The activators can be arranged at lattice sites orinterstitial lattice sites of the host lattice.

[0046] As a host lattice are preferred fluorides e.g. fluorides ofcomposition M¹F with M₁=Li, Na, K, Rb, Cs or fluorides of compositionM²F₂ with M²=Mg, Ca, Sr, Ba or fluorides of composition M³F₃ with M³=B,Al, In, Ga, Sc, Y, La and lanthanoids. Particularly preferred is GdF₃ inwhich the first lanthanoid activator ion Gd³⁺ is part of the hostlattice.

[0047] Furthermore as host lattices are preferred ternarygadolinium-containing fluorides of the composition M¹GdF₄, M¹ ₂GdF₅, M¹₃GdF₆, M¹Gd₂F₇, M¹Gd₃F₁₀, M¹ ₅Gd₉F₃₂, with M₁=Li, Na, K, Rb, Cs orM²GdF₅, M² ₂GdF₇, M² ₃GdF₉, M²Gd₂F₈, M²Gd₃F₁₁, M²Gd₄F₁₄, M² ₁₃Gd₆F₄₃with M²=Mg, Ca, Sr, Ba, Mn, Zn in which gadolinium is also part of thehost lattice.

[0048] Also preferred as host lattices are fluorides of the compositionM¹M³F₄, M¹ ₂M³F₅, M¹ ₃M³F₆, M¹M³ ₂F₇, M¹M³ ₃F₁₀, M¹ ₅M³ ₉F₃₂ with M₁=Li,Na, K, Rb, Cs and with M³=B, Al, In, Ga, Sc, Y, La, and the lanthanoids;M²M³F₅, M² ₂M³F₇, M² ₃M³F₉, M²M³ ₂F₈, M²M³ ₃F₁₁, M²M³ ₄F₁₄, M² ₁₃M³ ₆F₄₃with M²=Mg, Ca, Sr, Ba, Mn, Zn and M³=B, Al, In, Ga, Sc, Y, La, and thelanthanoids; M³M⁴F₇, M³ ₂M⁴F₁₀, M³ ₃M⁴F₁₃, M³M⁴ ₂F₁₁, M³M⁴ ₃F₁₅, M³M⁴₄F₁₉ with M³=B, Al, In, Ga, Sc, Y, La, and the lanthanoids, and M⁴=Ti,Zr, Si, Ge, Sn, Pb.

[0049] Particularly preferred as host lattices are fluorides of whichthe host lattice is based on the calcium fluoride crystal lattice type.In these matrices the cations have an 8-fold co-ordination. Alsoparticularly preferred are fluorides with a lattice derived from the YF₃crystal lattice type, in which the cations have a 9-fold co-ordination.Due to the high co-ordination figures and the non-polar ligands, thesehost lattices are characterized by a low ligand field for cations whichare part of the host lattice.

[0050] The luminophores doped with the activator pair contain preferably10 to 99.8 mol % of the trivalent Gd³⁺ and 0.01 to 30 mol %,particularly preferably 1.0 mol % of the trivalent holmium or trivalenteuropium.

[0051] The down conversion luminophore can easily be doped with thetrivalent co-activators terbium, ytterbium, dysprosium, europium,samarium or manganese if, in the production of the luminophores, to thestarting compounds is added a fluoride selected from the group TbF₃,YbF₃, DyF₃, EuF₃, SmF_(e) or MnF₂.

[0052] The absorption co-efficient of the down conversion luminophoressensitized according to the invention is particularly large for thewavelengths in the range of xenon radiation, and the quantum efficiencylevels are high. The host lattice is not a factor in the luminescenceprocess but influences the precise position of the energy level of theactivator ions and consequently the wavelengths of absorption andemission. The emission bands lie in the range from long ultraviolet toyellow-orange, but mainly in the red and green range of theelectromagnetic spectrum. The extinction temperature of theseluminophores is above approximately 100° C.

[0053] The grain size of the luminophore particles is not critical.Normally the luminophores are used as fine grain powders with a grainsize distribution between 1 and 20 μm.

[0054] As a production process for luminophore layers on a wall of thedischarge vessel both dry coating processes e.g. electrostaticdeposition or electrostatic-supported sputtering, and wet coatingprocesses e.g. dip coating or spraying, can be considered.

[0055] For wet coating processes the luminophore preparation must bedispersed in water, an organic solvent where applicable together with adispersion agent, a tenside and an anti-foaming agent or a binding agentpreparation. Suitable binding agent preparations for a gas dischargelamp according to the invention are organic or inorganic binding agentswhich tolerate an operating temperature of 250° C. without destruction,embrittlement or discoloration.

[0056] For example the luminophore preparation can be applied to a wallof the discharge vessel by means of a flow coating process. The coatingsuspensions for the flow coating process contain water or an organiccompound such as butylacetate as a solvent. The suspension is stabilizedand its rheological properties influenced by the addition of additivessuch as stabilizers, liquifiers, cellulose derivatives. The luminophoresuspension is applied to the vessel walls as a thin layer, dried andburned in at 600° C.

[0057] It can also be preferred that the luminophore preparation for theluminophore layer is deposited electrostatically on the inside of thedischarge vessel.

[0058] For a gas discharge lamp which is to emit white light, preferredsubstances are a blue-emitting luminophore from the groupBaMgAl₁₀O₁₇:Eu²⁺ and Sr₅(PO₄)₃Cl: Eu²⁺ with a red-emitting luminophoreaccording to the invention or from the group (Y,Gd)BO₃:Eu³⁺ andY₂O₃:Eu³⁺ and a green-emitting luminophore according to the invention orfrom the group Zn₂SiO₄:Mn²⁺; (Y,Gd)BO₃:Tb; CeMgAl₁₁O₁₉:Tb andLaPO₄:Ce,Tb or with a green-red-emitting luminophore according to theinvention.

[0059] The luminophore layer usually has a layer thickness of 5 to 100μm.

[0060] The vessel is then evacuated to remove all gaseous contaminantsin particular oxygen. The vessel is then filled with xenon and sealed.

EXAMPLE 1

[0061] A cylindrical glass discharge vessel with a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinternal electrode in the form of a noble metal rod of 2.2 mm diameter.On the outside of the discharge vessel is the external electrode of twostrips of conductive silver 2 mm in width arranged axis-parallel andconnected conductively with the power supply. The lamp is operated withpulsed DC voltage.

[0062] The inner wall of the discharge vessel is coated with aluminophore layer.

[0063] The luminophore layer contains a three-band luminophore mixturewith the following components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component,LaPO₄Ce,Tb as the green component and LiGdF₄:Eu,Pr as the red component.

[0064] To produce the LiGdF₄:Eu,Pr with 1.0 mol % europium and 1.0 mol %praseodymium, 29.4 g GdF, 3.6 g LiF, 0.29 g EuF₃ and 0.28 g PrF₃ werethoroughly mixed and ground in an agate mortar. The mixture waspreburned in a corundum crucible in a quartz tube under atmosphere ofargon with a pressure of 8 hPa for 2 hours at 300° C. During burning thequartz tube was flushed with argon three times and evacuated again to 8hPa. The oven temperature was then increased at a rate of 5.5° C. permin to 700° C. and the mixture sintered for 24 hours at 700° C. Thesintered powder was reground and sieved to a grain size <40 μm. Thecrystal structure of the formed phase was checked with X-raydiffractometry.

[0065] With this a light efficiency of initially 37 lm/W was achieved.After 1000 operating hours the light efficiency was 34 lm/W. The quantumefficiency for VUV light is approximately 70%.

EXAMPLE 2

[0066] A cylindrical glass discharge vessel with a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinternal electrode in the form of a noble metal rod of 2.2 mm diameter.On the outside of the discharge vessel is the external electrode of twostrips of conductive silver 2 mm in width arranged axis-parallel andconnected conductively with the power supply. The lamp is operated withpulsed DC voltage.

[0067] The inner wall of the discharge vessel is coated with aluminophore layer.

[0068] The luminophore layer contains a three-band luminophore mixturewith the following components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue componentand BaGdF₅:Er, Ho, Tb as the green-red component.

[0069] To produce the BaGdF₅:Er, Ho, Tb with 3.0 mol % erbium, 1.0 mol %holmium and 1.0 mol % terbium, 28.5 g GdF₃, 24.5 g BaF₂, 1.6 g ErF₃,0.31 g HoF3 and 0.30 g TbF₃ were thoroughly mixed and ground in an agatemortar. The mixture was preburned in a corundum crucible in a quartztube under atmosphere of argon with a pressure of 8 hPa for 2 hours at300° C. During burning the quartz tube was flushed with argon threetimes and evacuated again to 8 hPa. The oven temperature was thenincreased at a rate of 5.5° C. per min to 700° C. and the mixturesintered for 24 hours at 700° C. The sintered powder was reground andsieved to a grain size <40 μm. The crystal structure of the formed phasewas checked with X-ray diffractometry.

[0070] With this a light efficiency of initially 37 lm/W was achieved.After 1000 operating hours the light efficiency was 34 lm/W. The quantumefficiency for VUV light is approximately 70%.

EXAMPLE 3

[0071] A cylindrical glass discharge vessel with a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinternal electrode in the form of a noble metal rod of 2.2 mm diameter.On the outside of the discharge vessel is the external electrode of twostrips of conductive silver 2 mm in width arranged axis-parallel andconnected conductively with the power supply. The lamp is operated withpulsed DC voltage.

[0072] The inner wall of the discharge vessel is coated with aluminophore layer.

[0073] The luminophore layer contains a three-band luminophore mixturewith the following components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component,LaPO₄:Ce,Tb as the green component and NaGdF₄:Yb,Eu as the redcomponent.

[0074] To produce the NaGdF₄:Yb,Eu with 1.0 mol % ytterbium and 1.0 mol% europium, 29.1 g GdF₃, 5.9 g NaF, 10.35 g YbF₃ and 0.29 g EuF₃ werethoroughly mixed and ground in an agate mortar. The mixture waspreburned in a corundum crucible in a quartz tube under atmosphere ofargon with a pressure of 8 hPa for 2 hours at 300° C. During burning thequartz tube was flushed with argon three times and evacuated again to 8hPa. The oven temperature was then increased at a rate of 5.5° C. permin to 700° C. and the mixture sintered for 24 hours at 700° C. Thesintered powder was reground and sieved to a grain size <40 μm. Thecrystal structure of the formed phase was checked with X-raydiffractometry.

[0075] With this a light efficiency of initially 37 lm/W was achieved.After 1000 operating hours the light efficiency was 34 lm/W. The quantumefficiency for VUV light is approximately 70%.

1. A gas discharge lamp fitted with a gas discharge vessel filled with agas filling suitable for a gas discharge which emits VUV radiation, witha luminophore coating containing a down conversion luminophore, and withmeans for igniting and maintaining a gas discharge, in which the downconversion luminophore contains in a host lattice a pair of activatorsof a first lanthanoid ion and a second lanthanoid ion and a sensitizerselected from the group of the cerium (III) ion, praseodymium (III) ion,neodymium (III) ion, samarium (III) ion, europium (III) ion, gadolinium(III) ion, terbium (III) ion, dysprosium (III) ion, holmium (III) ion,erbium (III) ion, thulium (III) ion, ytterbium (III) ion and lutetium(III) ion.
 2. A gas discharge lamp as claimed in claim 1, characterizedin that the first lanthanoid ion is the gadolinium (III) ion and thesecond lanthanoid ion is selected from holmium (III) and europium (III).3. A gas discharge lamp as claimed in claim 1, characterized in that thedown conversion luminophore has as the first lanthanoid ion thegadolinium (III) ion and as the second lanthanoid ion the holmium (III)ion or the europium (III) ion and a co-activator selected from the groupof the terbium (III) ion, ytterbium (III) ion, dysprosium (III) ion,europium (III) ion, samarium (III) ion and manganese (II) ion.
 4. A gasdischarge lamp as claimed in claim 1, characterized in that the hostlattice of the down conversion luminophore is a fluoride.
 5. A gasdischarge lamp as claimed in claim 1, characterized in that the downconversion luminophore contains the first lanthanoid ion in aconcentration of 10 to 99.98 mol %, the second lanthanoid ion in aconcentration of 0.01 to 30 mol % and the sensitizer in a concentrationof 0.01 to 30.0 mol %.
 6. A gas discharge lamp as claimed in claim 1,characterized in that the down conversion luminophore contains thesensitizer in a concentration of 0.5 mol %.
 7. A gas discharge lamp asclaimed in claim 1, characterized in that the down conversionluminophore contains the co-activator in a concentration of 0.01 to 30mol %.
 8. A gas discharge lamp as claimed in claim 1, characterized inthat the down conversion luminophore contains the co-activator in aconcentration of 0.5 mol %.
 9. A down conversion luminophore which in ahost lattice has a pair of activators of a first lanthanoid ion and asecond lanthanoid ion and a sensitizer selected from the group of thecerium (III) ion, praseodymium (III) ion, neodymium (III) ion, samarium(III) ion, europium (III) ion, gadolinium (III) ion, terbium (III) ion,dysprosium (III) ion, holmium (III) ion, erbium (III) ion, thulium (III)ion, ytterbium (III) ion and lutetium (III) ion.